Watch pieces comprising such a calendar display belong to the category of watch pieces referred to as “complicated time pieces” and allow the user to view, by a single glance at the dial of the watch piece, information relating to the day, date and month, more specifically with a view of the entire current month. Documents DE 25 267 and U.S. Pat. No. 340,855 are typical examples of this type of device. These devices afford the notable advantage of very conveniently providing the information of which day of the week corresponds to which date for an entire month. However, owing to the fact that the length of each of the months is not generally a multiple of 7, the length of a week, these devices pose the significant drawback that they require the manual intervention of the user at the start of each month in order to readjust the inscriptions on the scale of days with the inscriptions on the scale of dates, otherwise the calendar display would no longer be correct.
Within the field of date display and, in general, of displaying calendar data, mechanisms called “perpetual calendars” by the person skilled in the art are also known. This type of mechanism is normally pre-programmed by using cams representing the length of the months and makes it possible to display the day, the date, the month and possibly the year for long periods, sometimes more than 100 years. In the majority of cases perpetual calendar mechanisms are used to display the aforementioned data by means of a plurality of individual apertures arranged in the dial of a watch piece. This is also the case in the first embodiment of a watch described in document EP 1 351 104. This type of display only shows the calendar data for a single day and therefore does not afford the aforementioned advantage of providing a general view over an entire month. For this reason, the second embodiment of a watch described in document EP 1 351 104 proposes a calendar display device of the type mentioned at the outset by combining it with a perpetual calendar mechanism, thus adding the advantage of a general view of the calendar data over an entire month. Moreover, since it is equipped with a perpetual calendar mechanism, the device described in this document affords the advantage that the inscriptions on the scale of days are automatically readjusted at the end of each month with the inscriptions on the scale of dates.
However, the reader notices by a detailed study of this document that these two advantages are only obtained as a result of two technical features which are rather difficult to implement and, technically, are quite complex in their design. In fact, the operation of the perpetual calendar according to EP 1 351 104 is based on a program wheel carrying a retractable tooth of which the movement is controlled by rather complicated kinematics arranged on this program wheel. Moreover, the kinematic chain of the movement according to EP 1 351 104 requires two drive trains controlling, in a coordinated yet separate manner, starting from a driving wheel, the rotation of the disk of months and that of the disk of dates in relation to the fixed dial indicating the days of the week.
Document EP 1 351 105, from the same proprietor as document EP 1 351 104, describes the same device from another viewpoint and in particular relates to the problem that the calendar display devices of the type mentioned at the outset, in spite of the advantage of providing a general view of calendar data over a given month, do not make it possible until the evening of the last day of the current month to view the calendar of the following month. It is even stated in the introduction of this document that the design of a device of this type based on a mechanical watch movement would be very difficult.
Consequently, document EP 1 351 105 differs from the design of the calendar display devices of the type mentioned at the outset and suggests separating the disk of dates into two different disks each carrying approximately half the inscriptions of the dates from 1 to 31. The document further proposes turning these two disks relative to one another by rather complex kinematics already mentioned above depending on the date to be displayed, in such a way that the user can view the calendar data over a period corresponding to approximately one month, irrespective of whether it is the start or the end of the month. For example, in order to display a date at the start of the month, the first disk of dates carrying the dates from 1 to 15 is aligned with the second disk of dates carrying the dates from 16 to 31 in such a way that the numbers from 1 to 31 are arranged in ascending order, whereas in order to display a date towards the end of the month, the position of the first disk of dates carrying the dates from 1 to 15 is changed in relation to the second disk of dates carrying the dates from 16 to 31 in such a way that the numbers from 1 to 15 follow the numbers from 16 to 31, in either case the whole assembly being adapted to the dial indicating the days of the week.
This brief explanation demonstrates that the solution of documents EP 1 351 104 and EP 1 351 105, on the one hand in order to obtain an automatic adjustment of the scales of days of the week and that of the dates at the start of each month, and on the other hand in order to avoid the drawback of calendar display devices of the type mentioned at the start of this introduction of not allowing an extended view of the calendar data towards the end of the month, is rather complex in its design, is difficult to implement, is expensive to manufacture and partially even removes, owing to the fact of using two separate disks for the date, the main benefit and aesthetic attractiveness of this type of device.
Apart from the devices according to the above-mentioned documents, which however have major drawbacks as explained above, the prior art does not yet appear to include a mechanism which makes it possible, with the aid of relatively simple means, to provide the two aforementioned advantages in a calendar display device of the type mentioned at the outset, that is to say, on the one hand, an automatic indexing of the scales of days and dates at the start of each month and, on the other hand, the possibility of an extended view of the calendar data at any moment, even towards the end of the month. In view of the prior art currently known, there is thus a need to create such a device which makes it possible to provide these advantages using means which are simpler in design, easier to produce and less costly to manufacture.